014-Osmoregulation (1)Biology of Fishes

8/17/2019 014-Osmoregulation (1)Biology of Fishes

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BIOLOGY OF FISHES

FISH/BIOL 311

FORM AND FUNCTION, OSMOREGULATION: WATER

AND IONIC BALANCE IN DIVERSE AQUATIC

ENVIRONMENTS

General topics:

1. Definitions: the Laws of Diffusion and Osmosis

2. Functional components of the vertebrate kidney

3. Osmoregulation in freshwater fishes4. Osmoregulation in marine fishes

5. Osmoregulation in diadromous and euryhaline fishes

6. Osmoregulation in elasmobranch fishes

7. Osmoregulation in tetrapods


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1. DEFINITIONS: THE LAWS OF

DIFFUSION AND OSMOSIS

Diffusion:

The movement of ions and molecules through a medium, from aregion of high concentration to a region of low concentration.


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Osmosis:

The movement of water across a semi-permeable membrane:

when the concentration of solutes (in this case glucose) is greater on

one side of the membrane than the other, the net movement of waterwill be from the region of lesser concentration to region of greater

concentration of solutes.


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For the most part, marine invertebrates are in osmotic equilibrium with

the seawater. That is, their salty internal fluids hold as much salts as does

the surrounding aquatic medium.

Stated another way, the principal ions that are found in the fluids that bathe

the cells of the body are the same, and occur in approximately the same

concentrations, as those found in seawater.

There is no problem of water balance—the rate of diffusion of water into the

body is the same as the rate at which water diffuses out. Under conditions

such as this, we say the animals exist in an isotonic environment (iso means

“the same”).


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We assume that this isotonic situation is the way it was for the chordate

ancestor of the vertebrates as well as for the earliest of vertebrates that livedin marine environments: the primitive kidney of these organisms functioned

solely to rid the body of waste materials —it had little or nothing to do with

water and salt balance. In other words, it was excretory in function rather

than osmoregulatory.


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In these forms, osmotic equilibrium was

disrupted because of the Law of Osmosis:

any animal submerged in freshwater with

body fluids in greater concentration than

the surrounding water inevitably takes inexcess water. That is, it tends to become

waterlogged either by:

1. Absorption through the delicate

epithelium covering the gill filamentsand mucous membranes of the mouth

and pharyngeal cavity, and by

2. Swallowing water along with food.

But what about those early

vertebrates that took to freshwater?


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So, very early on in the evolutionary history of vertebrates there was a

need for some kind of mechanism to rid the body of excess water.

Section through a human kidney

At the same time, salts are scarce

in freshwater environments, the

only source being food. Therefore,

because of the Law of Diffusion,

there was also a need for somemechanism to prevent the loss of

salts from the body.

What evolved to take care of these

problems was the vertebratekidney, also called the glomerular

kidney. A device that functions to

both eliminate excess water from

the body and to reclaim salts.


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2. FUNCTIONAL COMPONENTS OF

THE VERTEBRATE KIDNEY

1. Glomerulus

2. Convoluted or nephric tubule

3. Longitudinal collecting duct

The vertebrate kidney is made up of thousands of individual tubularstructures called nephrons. Each nephron has three components:


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The glomerulus is a tuft of blood capillaries surrounded by a capsule of tissue

called Bowman’s capsule (named for British surgeon and anatomist William

Bowman, 1816–1892). Together the glomerulus and capsule are called a renal

corpuscle.

The glomerulus, with the help of blood pressure, functions to filter water and

certain other fluid substances out of the blood stream. The filtered substances

are called the glomerular filtrate. Supplying each glomerulus is an afferent

renal arteriole and an efferent renal arteriole.

Bowman’s capsule

Glomerulus

Afferent renal arteriole

Efferent renal arteriole


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The convoluted tubule, differentiated into a number of discrete segments, is a

duct that collects the glomerular filtrate from the glomerulus and transports it

to the longitudinal collecting duct.

All along the length of each tubule many substances are reabsorbed by the

blood by means of a second capillary bed that surrounds the tubule. This

capillary network is supplied by an afferent renal venule and drained by an

efferent renal venule. Other substances may be secreted into the tubule from

the blood.

Distal convoluted

tubule

Proximal convoluted

tubule

Afferent renal venule

Efferent renal venule


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The Longitudinal collecting duct receives all the glomerular filtrate

from the thousands of tubules of the kidney and dumps it into the urinary

bladder.

Longitudinal collecting duct


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Critical to the functioning of

each nephron are two sets ofcapillaries:

Glomerular mass embeddedwithin Bowman’s capsule,which, with the aid of blood

pressure, acts as a filter toremove excess water andother substances, and a

Reabsorptive mass that

surrounds the convolutedtubule and functions toreabsorb water and othervaluable substances, whileit reabsorbs or excretes salts.

Glomerularmass

Reabsorptive mass


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So this is the vertebrate or

glomerular kidney, a wonderfuldevice that takes care of the water

problem and, at the same time,

functions to retain glucose, salts,

and other valuable materials by

reabsorption through an elaboratesystem of nephric tubules. The

basic structure is rather simple, but

it’s important to realize that there is

huge variation among fishes (and

vertebrates in general) in the number,

size, complexity, and arrangement of

the glomeruli and tubules.

The human nephron


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3. OSMOREGULATION IN A

FRESHWATER FISH

Let’s turn now to the fishes themselves and see how the problems ofosmoregulation compare in freshwater habitats and in marine habitats.

A fish submerged in freshwater with body fluids in greater concentrationthan the surrounding water tends to take on water and lose valuable salts tothe environment. It takes on water primarily by absorbing it through the gills

and skin by simple passive diffusion.

Under these conditions the animal exists in a hypotonic environment (hypomeans “less” referring to the lower solute concentration of the surroundingwater). It must be able to eliminate excess water and retain salts. How does

it do this?


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1. Drinks very little water 2. Has numerous, large, well-developed glomeruli

3. Reabsorbs salts along the length of its convoluted tubules

4. Produces large amounts of very dilute urine (5-12% of body

weight per day).


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To better understand how osmoregulation

works in fishes living in diverse aquatic

habitats, it’s necessary to take a more

detailed look at the functional

components of the kidney.

As an example, let’s look closely at the

kidney of a common freshwater fish,

the carp (Cyprinus carpio), which has a

kidney that is as complex as a fish kidney

gets—an almost identical pattern is found

in amphibians.

Excess water is filtered from the bloodthrough the glomeruli carrying all kinds

of substances in addition to the water,

such as salts and sugars, which are

reabsorbed into the blood stream through

the epithelium of the kidney tubules.


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Glomerulus: A typical kidney of a

freshwater fish has thousands of large

glomeruli, each with a well-developed blood supply. Great amounts of water

pass through them. The glomerulus,

then, is a device that provides a filtrate

that can be modified selectively by the

kidney tubule.

Neck Region: The neck region is lined

with cilia. The ciliary action plays an

important role in aiding movement of

materials into the tubule. This is

particularly important in the low-

pressure filtration systems of fishes.

Neckregion


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First Proximal Segment: Here is

where reabsorption of many macro-molecules, such as glucose and proteins

takes place, but also excretion of toxic

organic acids.

Second Proximal Segment: This is

the largest region of the tubule, where

there is high metabolic activity, i.e.,

active transport mechanisms that are

responsible for the reabsorption of

many salts, such as Mg++, SO4--, Ca++,

P, Na+, Cl-, and HCO3-


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Distal Segment: this portion of

the tubule participates in activereabsorption of Na+ and some Cl- ions.

It is also a highly ciliated area that

assists in propulsion of fluid along the

tubule. In a freshwater fish it is

important to move the fluid throughthe length of the tubule as fast as

possible to minimize passive

reabsorption of water.

Collecting Tubule or Duct: functions primarily to reabsorb monovalent ions,

mostly Na+ and Cl-.


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What is left is a dilute urine that contains

mostly water, but also some creatine and

creatinine (alkaloids), some amino acids,

and a little urea and ammonia.

Some nitrogenous waste is lost by way

of the urine, but this amounts to only 7to 25 % of the total nitrogen excreted by

a freshwater fish. The bulk passes out

through the gills in the form of ammonia.

The remainder is mostly urea and othersimple compounds of nitrogen that also

leave the body by way of the gills.

Dilute urine


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The kidney alone cannot reabsorb enough salts to maintain osmoregularity.

To compensate for this deficiency the gills and oral membranes have evolved

the ability to absorb ions by active transport mechanisms in special cells

called chloride cells. All kinds of ions are reabsorbed in this way: acid

phosphate (HPO4-), bromine (Br -), calcium (Ca++), chloride (Cl-), lithium (Li+),sodium (Na+), sulfate (SO4

--) ions, etc.


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4. OSMOREGULATION IN MARINE FISHES

Marine fishes have problems too. Their body fluids, although consistingof the same ions as seawater, have a total quantity of salts that is less than

that of the same volume of seawater (some marine teleosts have as littleas one-third the osmotic concentration of seawater).

Because their body fluids are less concentrated than seawater they tend tolose water through their membranes. Under these conditions we say the

animal exists in a hypertonic environment (hyper means “more” referring

to the higher solute concentration of the surrounding water). They are forced

by osmotic conditions to conserve water and to get rid of excess salts.

How do they do it?


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1. Drink seawater 2. Have fewer and smaller glomeruli

3. Excrete salts along the length of their convoluted tubules

4. Produce small amount of very concentrated urine (as little as 2.5

ml per kg of body weight per day)


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Nearly all marine bony fishes

show a reduction in the number

and size of glomeruli, culminating

in some forms that have lostglomeruli. In addition to their

ability to produce a highly

concentrated urine, specialized

tissues in the gill region have

evolved to actively excretelarge amounts of salt.

Glomerulus: The glomeruli

of marine teleosts are small,

poorly vascularized, and blood

pressure in the glomeruli is low.

Forms in which glomeruli are few,

small, and degenerate are called

pauciglomerular.


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Some species thrive with no glomeruli

at all—these are called aglomerular.

Examples include the midshipmen(genus Porichthys) and the goosefish

(genus Lophius).


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Neck Region: This region may be

lost altogether, especially in the case

of aglomerular species.

First Proximal Segment: Here,

just as in freshwater fishes, there is

reabsorption of macromolecules

such as glucose and proteins.

Second Proximal Segment: Instead

of active reabsorption of many salts

as we saw in freshwater fishes, this

part of the nephron is a site of active

secretion of salts, such as Mg++, SO4-,Ca++, P, Na+, Cl-, and HCO3

-. It is

also responsible for active secretion

of nitrogenous waste produce like

urea, creatinine, and creatine.


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Distal Segment: This portion,

which in freshwater forms is

heavily ciliated and assists in propelling fluid along the tubule,

is absent in marine fishes. The

requirement here is to slow the

movement of fluid so that there

is time for the maximum amountof passive diffusion of water

back into the blood.

Collecting Tubule or Duct:

participates in some reabsorption

of Na+ and Cl- ions.


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What’s left is a small volumeof highly concentrated urine,

containing creatinine, creatine,

some urea and some ammonia,

plus other miscellaneous

nitrogenous compounds.

But 90 percent of the

nitrogenous waste products

is not excreted by the kidneys, but eliminated by the gills as

ammonia and urea.Highly

concentrated

urine


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Again, just as in freshwater fishes,

the gills are very important in ionic

balance. The kidneys alone cannot

eliminate all the excess salts.Whatever they can’t handle is

excreted by the gills so that the bulk

of monovalent ions, especially

chloride ions, pass out through the

gills.

This is done by a process of active

transport that takes place in the

special secreting cells called

chloride cells —not such a goodname, because they are responsible

for the secretion of other ions as

well. These chloride cells are rich

in mitochondria —a site of great

metabolic activity.


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5. OSMOREGULATION IN DIADROMOUS

AND EURYHALINE FISHES

So far we have been talking only about fishes that live strictly in either

freshwater or salt water, that is, fishes that have a very narrow tolerance to

salt. These are called stenohaline forms (from the Greek stenos, meaning

narrow; and the Greek hals or halos, meaning salt or sea).

Many fishes, however, have a wide tolerance to salt and can live in

freshwater, brackish water, or salt water, and can move freely among these

different habitats. These are called euryhaline forms (from the Greek eury,

meaning broad or widespread).

Some species have split life histories, spending part of their lives in

freshwater, the other part in the marine habitat. These are called

diadromous (from the Greek di, means two, and dromos refers to running).


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Chinook Salmon (Oncorhynchus tshawytscha)

Diadromy takes three general forms:

1. Anadromy: Adults spawn in freshwater; juveniles move to

saltwater for several years of feeding and growth, and then migrate back to freshwater to spawn (from the Greek ana, meaning “up,” to

run up-stream to spawn).


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2. Catadromy: Adults spawn at sea; juveniles migrate to freshwater for

several years to feed and then return to the sea to spawn (from the Greekkata, meaning “downward,” to run down-stream to spawn).

American eel ( Anguilla rostrata)


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3. Amphidromy: Spawning may occur in either fresh or saltwater; larvae

migrate to the other habitat for initial feeding and growth, then migrate to the

original habitat as juveniles or adults, where they remain for additional feeding

and growth prior to spawning (from the Greek amphi, meaning “both sides”).


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Three forms of diadromy: B = birth, G = growth, and R = reproduction


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6. ELASMOBRANCH FISHES

Marine elasmobranchs have solved the problem of osmoregulation in an

entirely different way. They have evolved a specialized segment of the

nephron that reabsorbs urea and returns it to the blood.

Osmoregulation in a marine shark. The concentration of solutes in the body fluids is greaterthan in the outside medium. Open arrows indicate movement of substances by

passive diffusion, closed arrows indicate movement of substances

by active transport mechanisms


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This influx of urea, a toxic

nitrogenous waste producefor most vertebrates, raises the

osmotic pressure of the blood

to a level just above that of sea

water so that water actually

flows into the body of theshark.

Marine sharks thus act like

freshwater fishes: they have

numerous well-developedglomeruli and they excrete

large amounts of dilute

urine. Dilute urine


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7. OSMOREGULATION IN TETRAPODS

Terrestrial vertebrates also have water conservation problems. Like marine

teleosts, the kidneys of reptiles and birds have very reduced glomeruli,although no aglomerular kidneys are found. As a further aid, water is

reabsorbed by the walls of a cloaca. The result is very dry feces consisting

primarily of uric acid. In addition, birds have a specialized segment of the

nephron that reabsorbs water.

Marine turtles, the marine iguanas of the Galapagos, and marine birds

have solved the problem in slightly different ways.


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Desalting of the water and retention of freshwater is accomplished by

special salt excreting glands in the head. Salt is dumped into ducts

that empty into the nasal cavities or directly to the outside.

Marine turtles, the marine iguanas of the Galapagos, and marine

birds drink seawater.


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In mammals, a large section of

the nephric tubule has become

adapted solely to reabsorb water.

The glomerulus actually allows

more than 100 times the amount

of water to filter from the blood as

would be excreted in the urine that

leaves the other end of the tubule.

Nearly all of this extra water is

reabsorbed or pushed back into

the blood by the activity of this

elongate tubule called the Loop

of Henle, named after German

physician Friedrich Gustav Henle

who first described it in 1841.The human nephron

Loop of Henle

014-Osmoregulation (1)Biology of Fishes

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